Polyester resins, and among these particularly alkyd resins are one of the most common binder class used for ambient-cure, solvent-based coatings. The resistance properties of traditional solvent-borne alkyd resins are developed via autooxydative crosslinking of the paint film when subjected to air. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidised in air to the corresponding hydroperoxides which subsequently decompose to generate free radicals, thus resulting in an oxydative crosslinking process. This oxydative crosslinking process is usually accelerated by adding the so-called driers, such as, for example, various salts of cobalt, zirconium, calcium, and manganese. However, alkyd resins have relatively slow “dry” or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes modification of a polyester resin or an alkyd resin with a vinyl or acrylic polymer, based on monomers such as styrene or methyl methacrylate, via a free radical polymerisation of these monomers in the presence of the alkyd resin, to produce a vinyl-alkyd copolymer or a so-called “vinylated alkyd” or “vinylated polyester”.
Vinylated alkyd resins generally have a higher molar mass and a higher glass transition temperature (Tg), leading to coatings with reduced tack-free time. However, the so-called “through-dry time” (complete oxydation of unsaturation in the fatty acids forming the alkyd part of the modified resin) of such coatings is longer than in non-modified alkyds due to the decreased degree of unsaturation in the alkyd as a result of copolymerisation with the vinyl monomers. An additional drawback is that paint formulations containing vinylated alkyd resins require greater amounts of solvent, due to the increased molar mass and Tg of the vinylated alkyd.
Water-borne alkyd resins are applied in the form of aqueous emulsions or dispersions, and only need small quantities of coalescing additives. Evaporation of water and oxydative drying of the alkyd, however, occur on the same time scale as in solvent-borne alkyds. While other resin systems allow faster curing, the favourable properties of alkyd resins particularly on wood substrates, such as wood wetting (“Anfeuerung”), make the use of alkyds in this application field highly desirable.
In the published patent application WO 2003/087244 A1, a waterborne acrylate-functionalised alkyd coating composition is described which comprises an acrylate-functionalised alkyd resin, at least one drier, and water. The acrylate-functionalised alkyd resin comprises the reaction product of (a) an alkyd resin, (b) an acid anhydride, and (c) a glycidyl acrylate. The hydroxy end groups of the alkyd are converted to acid end groups by reaction of the alkyd resin with an acid anhydride to produce a carboxyl-functional resin, which in turn is then reacted with the glycidyl acrylate. A method of preparing a waterborne acrylate-functionalised alkyd coating composition is also disclosed comprising the step of contacting an acrylate-functionalised alkyd resin with at least one drier in the presence of water. Curing of these systems by irradiation has not been mentioned in this document.
Under the reaction conditions during the reaction of the hydroxy-functional alkyd resin with the acid anhydride, with the catalyst used for the synthesis of the alkyd resin still present, a certain amount of transesterification occurs, and if an acid-functional anhydride such as the trimellitic anhydride of WO 2003/087244 A1 is used, chain-branching will also be induced. The need for two consecutive reactions on the polymer is also a drawback. The unsaturation introduced by this process can be calculated from the amount of glycidyl acrylate (M=128.13 g/mol) corresponding to a mass fraction of from 3% to 12% in the resin, to be from 234 mmol/kg to 936 mmol/kg.
In the published patent application WO 2004/009 716 A1, alkyd-based free radical curable compositions are disclosed wherein the polymer comprises an alkyd portion, and a chain-pendant free radical curable portion which contains ethylenically unsaturated groups. These groups are connected to the polymer chain preferably via an isocyanate-functional ethylenically unsaturated compound, which is preferably a reaction product of a diisocyanate with a hydroxy-functional (meth)acrylic ester such as hydroxyethyl methacrylate, or of a diisocyanate with a hydroxyalkyl vinyl ether such as 4-hydroxybutyl vinyl ether. The hydroxy functional alkyd resins are prepared by converting the majority of the acid groups in an alkyd resin to hydroxyl groups by first reacting with an epoxide, such as neodecanoyl glycidyl ester. Modification of the alkyd resins as described here comprises three steps starting from a pre-formed alkyd: firstly, conversion of acid groups to hydroxyl groups by reaction with an epoxy compound, secondly, preparation of a functionalising agent by reacting a di-isocyanate with a hydroxy-functional ethylenically unsaturated compound, and thirdly, reaction of the hydroxy functional polyester or alkyd with the isocyanate functional unsaturated compound. The curable compositions described herein are solvent-borne.
It is therefore an object of the invention to provide a water-borne radiation-curable composition based on polyester resins or alkyd resins having olefinic unsaturation.
This object has been realised by providing a modified polyester or alkyd resin made by reaction of a hydroxy functional polyester or alkyd resin with an olefinically unsaturated compound having reactive groups which are reactive towards hydroxyl groups in the said polyester or alkyd resin, and by dispersing this modified polyester or alkyd resin in an aqueous medium using an emulsifier which is compatible with the said modified polyester or alkyd resin, to form a radiation-curable composition which can be radically polymerised following irradiation with high-energy radiation such as ultraviolet light or electron beams.